This invention relates to techniques for chamfering layers or plies, herein commonly referred to as sheets, used in composite structures, such as wind turbine blades.
Composite structures typically comprise one or more sheets, herein also referred to as plies, each ply being a fibre-reinforced sheet that may comprise a thermoplastic or thermosetting resin matrix. The fibres may be pre-impregnated with the matrix as a ‘prepreg’ or the matrix may be impregnated into a fibre sheet during fabrication of a composite structure, for example during lay-up or injection-moulding procedures. Alternatively, the fibre-reinforced sheet may be pre-impregnated on just one side by a resin foil, i.e. a ‘semi-preg’. As a further alternative, the fibrous material can be embedded with the laminate matrix resin by a vacuum assisted transfer into a dry fibrous material layup, e.g. such as in VARTM (Vacuum assisted resin transfer molding). The resin normally used is a thermosetting resin, which drops in viscosity when heated and later rises to become solidified, when curing at continued heating.
Plies are commonly laid atop one another in a layered or laminated arrangement. Single-ply composite structures are also possible, with single-thickness plies overlapping at their edges. The plies can be supported by a foam core to define a skin on or around the core, e.g. to provide a sandwich structure.
In wind turbine blades, the structure is usually tapered in both the spanwise direction from blade root to blade tip and in the chordwise direction. To achieve this, some plies may be terminated or ‘dropped’ inward of an extremity of the structure, leaving other continuous plies to extend further toward that extremity. However, such ply-drops may cause weaknesses in the laminate, in turn causing damage such as delamination or cracks. Edge chamfering is helpful to straighten the load path and to maximise the surface area of the interface between plies. This allows thicker plies to be used, which facilitates the lay-up process because fewer layers are then required in the laminate to achieve a required overall thickness.
Plies for use in composite structures are difficult to chamfer efficiently, accurately and repeatably, particularly with the shallow taper angle that is desirable to maximise the surface area of the edge interface. The plies are flexible and compressible and so tend to move unpredictably under the forces applied by the chamfering process. Also, the plies may degrade with heat generated by the chamfering process. This is a particular problem with prepregs, if the matrix cures or otherwise transforms with heat. For example, heat generated during chamfering may cause the thermoplastic matrix to soften or melt and clog the chamfering tool. If the matrix softens or melts, it is also possible for the chamfering tool to drag the ply unpredictably, possibly distorting it and so undermining the accuracy of cutting.
Some examples of ply-tapering tools are disclosed in EP1786617. These include finger cutters akin to hair trimmers, but finger cutters are not suitable for cutting prepregs in which the fibres are embedded in a matrix because the close integration of the fibers in a prepreg ply prevents the fingers from penetrating between the fibres. Also, in sheets of dry fiber, in which the fibres are held together, e.g. by stitching, and oriented at various angles, the fingers would be prevented from penetrating between the fibres. EP1786617 also discloses milling cutters with inclined faces, turning about an axis orthogonal to a plane containing the edge being tapered. When configured as shown in EP1786617, milling cutters impart heat to a prepreg ply that may degrade the ply and melt its matrix; this is also a problem suffered by other abrading techniques, using a belt grinder or the like. Also, in a dry fiber ply, when configured as shown in EP1786617, milling cutters impart a side force to the ply, parallel to the tapered edge, that tends to distort the ply, and individual fibers in it, and so undermines the accuracy of cutting.
WO2012013193 discloses a technique where a prepreg ply is clamped between refrigerated steel blocks, a grinding wheel is arranged to translate across a free edge of the ply to remove material from that edge to create a chamfer, and a nozzle supplying refrigerant is arranged to move in tandem with the grinding wheel. Although this is a promising technique, there is still room for improvement, in particular to facilitate large scale manufacturing, e.g. of wind turbine blades. Hence, there is a need for a process for chamfering edges of fibrous material sheets in a controlled manner and in a consistent quality suitable for blade manufacturing in high numbers.